Optical fiber transmission systems are very widespread today and support very high speed data, audio and video transmission. Due to the increased importance and reliability levels that shall be supported, performance monitoring and management of networks become increasingly important. The need for reliable tools that are capable of detecting faults of the physical carrier, i.e. of the optical fibers, is increasingly felt. During implementation of fiber plants as well as afterwards during the network operation, it is necessary to apply a method to check the optical line condition, since a quick detection and identification of fiber link failures can help to minimise service downtime for the user and any loss of revenue to the network operator (provider).
As more fibers are deployed in metropolitan and access networks there is an increasing need for continuously or at least regularly supervising the performance of the optical links. Supervision enables preventive countermeasures in case of early detection of link degradations, thus securing a high availability of the network, e.g. for delivering critical services to business customers. Also in case of failures the introduction of supervising means helps to quickly localise and identify the cause of failure to initiate repair or restoration actions.
The network operator needs to quickly identify the cause of a link failure, i.e. to decide between the fiber plant, i.e. the optical fibers (fiber link) themselves, and the node equipment and, in case of fiber problems, to localise and identify the type of fault or degradation along the fiber link. Most fiber link problems are related to increased losses and reflections that either prevent from error free detection of the data or that disturb the emitting loser, causing distortions of the transmitted data. Problems due to changes of the chromatic dispersion are unlikely due to the low data rates considered in access. Therefore PON access network monitoring is a key for preventive maintenance and network reliability to be guaranteed by the operator of the PON.
Within access networks beside point to point (p-t-p) links passive optical distribution systems (PON) are of great interest. PON technology represents a cost effective architecture for a local loop mainly by eliminating complex and expensive active powered elements between a service provider and subscribers.
Continuous optical performance monitoring and supervision, detecting and localising faults, i.e. measuring characteristics of fiber links, in PONs in access are preferable network features increasing the service availability and providing substantial cost savings to the providers. PON networks are based on Optical Distribution Networks where splitters are located in the field outside the Central Office (CO), meaning a network centre, where an Optical Line Terminal is connected to the feeder section of the ODN.
In p-t-p networks Optical Time Domain Reflectometry (OTDR) techniques are used to monitor the network by launching an optical single pulse into a fiber link and measuring the reflected light enabling a characterisation of fiber optical links. It is difficult in a PON network to check the fiber link properties beyond a passive distribution node from the central office side because the responses, i.e. back scattered reflections, from all connected fiber link branches beyond the splitters are superimposed and a separation is not easily possible. The OTDR signal of each branch is partially masked by the signals of the others. Therefore, the use of existing conventional CO-based OTDR techniques for optical performance monitoring like the single pulse method is not applicable to achieve unambiguous results in PON-type optical distribution networks.
Furthermore, optical time domain reflectometry in passive optical networks can suffer from the significant splitting losses which are reducing the achievable measurement precision and are increasing the necessary measurement time. Especially PONs with distributed splitting location hinder reflectometric measurements of intermediate links, meaning fiber links between two splitters. This is of significant concern for measurement/monitoring techniques which use the optical link optical sources and detectors, meaning the sources and detectors of the transceiver devices of the ONUs and/or the ONTs, for monitoring purpose and cannot introduce high measurement signal power levels.
To mitigate this splitter attenuation problem, high peak output power optical sources are deployed in addition to the optical link emitters and coupled to the fiber links with dedicated extra optical switches or couplers. Basically high attenuation PON networks with distributed splitters are not monitored at this time, nor monitored continuously.
The basic problem to use OTDR technique in PON is the measuring at the distribution section, i.e. the fiber links after a splitter (distribution fibers), because of the attenuation of the forward signal, i.e. the exciting signal, as well as of the back scattered measuring signal. This attenuation causes the power level of the back scattered parts of a measuring signal emitted from OLT side to be not high enough for the desirable precise and sensitive measuring.
The known monitoring approaches cause considerable technical effort and high costs. The dedicated OTDR equipment is expensive and often has to be transported to the expected fault site on demand. The extra coupling devices introduce permanent additional insertion loss in the data link. No continuous monitoring is achievable.